1. Field of the Invention
The present invention relates to a method of making a resist pattern and more particularly, to a method of making a resist pattern which is applicable to formation of contact holes and/or via holes in a layer or layers of a substrate in semiconductor device fabrication, where the resist pattern (i.e., the patterned resist layer) is used as a mask in the contact/via hole formation process.
2. Description of the Related Art
Semiconductor devices comprise xe2x80x9ccontact holesxe2x80x9d for electrically interconnecting a specific semiconductor or conductive layer with a wiring layer and/or xe2x80x9cvia holesxe2x80x9d for electrically interconnecting an upper wiring layer with a lower wiring layer. In recent years, contact holes and via holes have been becoming smaller according to the increasing integration level.
Typically, contact/via holes are formed by the well-known photolithography and etching techniques. Specifically, after a photo-sensitive resist layer is formed on a dielectric layer, the resist layer thus formed is patterned to have desired openings. The resist layer thus patterned constitutes a xe2x80x9cresist patternxe2x80x9d having the openings. Thereafter, using the resist layer thus patterned (i.e., the resist pattern) as a mask, the underlying dielectric layer is selectively etched by way of the openings of the mask, forming contact/via holes in the dielectric layer. Thus, to miniaturize contact/via holes in a desired or target layer, the dimensions of the openings of the resist pattern (i.e., the mask) need to be made smaller according to the desired dimensions of the contact/via holes.
Moreover, since the dimensional error of the contact/via holes (which includes deformation of the contact/via holes themselves) varies dependent on the dimensional error of the openings (which includes deformation of the openings themselves) of the resist pattern, there is the need to form smaller openings in the resist pattern as precisely as possible.
As a result, conventionally, a great deal of effort has been put to decrease the dimensional error of the smaller openings of the resist pattern.
With an example of the prior-art methods to make smaller openings of the resist pattern, after original openings are formed in a resist layer, the resist layer is subjected to a heat treatment at a temperature higher than its softening point, thereby gradually causing plastic deformation in the resist layer. In this method, the dimensions of the original openings are decreased due to the plastic deformation while the shape of the original openings is well controlled. This method is disclosed, for example, in the Japanese Non-Examined Patent Publication Nos. 2-7413 published in 1990, 10-274854 published in October 1998, and 11-295904 published in October 1999.
FIGS. 1A to 1D show the prior-art method of making a resist pattern disclosed in the Japanese Non-Examined Patent Publication No. 11-295904.
First, as shown in FIG. 1A, a positive electron-beam (EB) resist with chemical amplification property (i.e., a positive, chemically amplified resist) is coated on the surface of a substrate 110, forming a resist layer 111 thereon. The target layer (not shown) in which contact/via holes are formed is located at the top of the substrate 110 and thus, it may be said that the layer 111 is located on the target layer.
Using an electron-beam direct writing apparatus, specific openings are formed or written in the resist layer 111. Thus, the layer 111 is selectively exposed to an irradiated electron beam, thereby forming desired exposition areas 111a in the layer 111. Desired original openings are formed in the individual areas 111a. 
Next, as shown in FIG. 1B, the resist layer 111 with the exposition areas 111a formed on the substrate 110 is subjected to a post exposure bake (PEB) process and developed, selectively removing the areas 111a from the resist layer 111. Thus, original openings 115 are formed to penetrate the resist layer 111 in the respective areas 111a. The resist layer 111 with the original openings 115 thus formed is termed an original resist pattern 112.
Subsequently, as shown in FIG. 1C, the original resist pattern 112 is subjected to a heat treatment at a specific temperature higher than the softening temperature of the resist layer 111 for reflowing the pattern 112. Through this reflowing process, the original openings 115 are plastically deformed and narrowed, resulting in narrowed or contracted openings 115a, as shown in FIG. 1D. The original resist pattern 112 with the narrowed or contracted openings 115a is termed a resultant resist pattern 113.
With the prior-art method explained with reference to FIGS. 1A to 1D, although the narrowing or contracting effect of the original openings 115 can be generated, there is a problem that unallowable fluctuation of the narrowing/contracting effect for the openings 115 occurs if the density of the openings 115 (i.e., the count of the openings 115 within a unit area) in the original resist pattern 112 varies locally within a wide range. This is due to the correlation of the deformation amount of the openings 115 with their density.
As a result, the resultant openings 115a thus narrowed or contracted tend to have an undesired shape and/or undesired dimensions. In the worst case, some of the original openings 115 disappear (i.e., cease to exist) and accordingly, desired contact/via holes are unable to be formed in the underlying target layer (not shown) of the substrate 110.
The above-described problem can be solved by a measure to change or adjust the dimensions of the original openings 115 in the original resist pattern 112 prior to the reflowing process according to the density distribution of the openings 115 within the pattern 112. This measure can be comparatively easily applied to the memory cells having the periodically-varying density distribution of the openings 115. However, this measure is very difficult to be applied to the logic cells having the randomly-varying density distribution of the openings 115.
The deformation amount of the individual original openings 115 for the logic cells can be predicted with the use of computer simulation or the like. In this case, however, there arises another problem that it takes extremely long time to make a desired resist pattern. Thus, the above-described measure is unable to be applied in practice.
Accordingly, an object of the present invention is to provide a method of making a resist pattern that decreases or eliminates the fluctuation of deformation of original openings of a resist layer which is induced by the change of their density (i.e., the count of the original openings within a unit area) in the reflowing process.
Another object of the present invention is to provide a method of making a resist pattern that decreases or eliminates the fluctuation of deformation of original openings of a resist layer which is induced by their location or position in the reflowing process.
Still another object of the present invention is to provide a method of making a resist pattern that prevents or effectively suppresses unallowable fluctuation of the narrowing/contracting effect for original openings of a resist layer even if their density varies locally within a wide range.
A further another object of the present invention is to provide a method of making a resist pattern that copes with the miniaturization of resultant openings with a simple measure.
A still further another object of the present invention is to provide a method of making a resist pattern that copes with the miniaturization of resultant openings with a simple measure independent of their density change.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
A method of making a resist pattern according to the present invention comprises the steps of:
(a) forming a resist layer on a target layer;
(b) patterning the resist layer to form original openings and at least one slit in the resist layer;
the slit surrounding at least one of the original openings and having a specific width; and
(c) reflowing the resist layer patterned in the step (b) under heat to cause deformation in the original openings and the at least one slit, thereby contracting the original openings and eliminating the at least one slit;
the original openings thus contracted serving as resultant openings for forming desired contact/via holes in the target layer;
the resist layer having the resultant openings constituting a resist pattern.
With the method of making a resist pattern according to the present invention, the resist layer, which is formed on the target layer, is patterned to form the original openings and the at least one slit in the resist layer in the step (b). The at least one slit surrounds the at least one of the original openings and has the specific width.
Thereafter, in the step (c), the resist layer is reflowed under heat to cause deformation in the original openings and the at least one slit, thereby contracting the original openings and eliminating the at least one slit. The original openings thus contracted serves as the resultant openings for forming the desired contact/via holes in the target layer. The resist layer having the resultant openings constitutes the resist pattern.
Accordingly, in the step (c), the at least one slit serves as if it is a breakwater for the softened, reflowing material of the resist layer with the original openings, thereby suppressing undesired, excessive deformation of the same material. Also, the at least one slit is eliminated in the step (c) and therefore, it gives no effect to the subsequent process steps.
When the at least one slit is provided to suppress or prevents the excessive deformation of the material of the resist layer in its peripheral area, the at least one slit is located between an edge of the resist layer and the original openings adjacent to the edge. In this case, relatively larger deformation of the material of the resist layer in the peripheral area with respect to deformation of the same material in the middle or internal area is suppressed effectively by the at least one slit. As a result, the fluctuation of deformation of the original openings of the resist layer which is induced by the positional change of the original openings is decreased or eliminated in the reflowing step (c).
Thus, unallowable fluctuation of the narrowing/contracting effect for the original openings is prevented or effectively suppressed even if some of the original openings exist in or near the peripheral area of the resist layer. This means that the method can cope with the miniaturization of the resultant openings with a simple measure.
On the other hand, when the at least one slit is provided to suppress or prevents the fluctuation of deformation of the material of the resist layer which is induced according to the density difference of the original openings, the at least one slit is located so as to surround the respective sets of the original openings arranged at different densities. Thus, the per-opening volumes of the material of the resist layer for the respective sets of the original openings are controlled to be approximately equal to each other. In this case, the deformation amounts of the material of the resist layer for the respective sets of the original openings are set approximately equal. As a result, the fluctuation of deformation of the original openings of the resist layer which is induced by their density change is decreased or eliminated in the reflowing step (c).
Thus, unallowable fluctuation of the narrowing/contracting effect for the original openings is prevented or effectively suppressed even if the density of the original openings varies locally within a wide range. This means that the method can cope with the miniaturization of the resultant openings with a simple measure independent of their density change.
In a preferred embodiment of the method according to the invention, the at least one slit is located between an edge of the resist layer and the at least one of the original openings adjacent to the edge.
In another preferred embodiment of the method according to the invention, first to n-th sets of the original openings are located at first to n-th densities in first to n-th regions of the resist layer, respectively, where n is an integer greater than unity. At least two of the first to n-th densities are different from each other. The at least one slit is formed to surround each of the first to n-th regions of the resist layer.
In this embodiment, preferably, the at least one slit is formed in such a way that per-opening volumes of a material of the resist layer in the first to n-th regions are approximately equal to each other.
In still another preferred embodiment of the method according to the invention, the at least one slit has a width in such a way as to disappear after the step (c) of reflowing the resist layer is finished.
In a further preferred embodiment of the method according to the invention, the resist layer is made of a positive photoresist material. For example, the material contains a quinone-azide-system photosensitive agent, an alkali-soluble resin, and a solvent.
Preferably, in the step (c) of reflowing the resist layer, the patterned resist layer is reflowed at a temperature higher than a softening temperature of a material of the resist layer by 10xc2x0 C. or more and not exceeding 150xc2x0 C. If the patterned resist layer is reflowed at a temperature higher than a softening temperature of a material of the resist layer by less than 10xc2x0 C., desired deformation does not occur in the resist layer. Thus, the advantages or effects of the invention are not given.
On the other hand, if the patterned resist layer is reflowed at a temperature exceeding 150xc2x0 C., excessive deformation occurs in the resist layer. Thus, the advantages or effects of the invention are not given.